A. Field of the Invention
This invention relates to the measurement of properties of liquids moving in a conduit and specifically the measurement of the flow rate of a liquid moving at a relatively low flow rate of less than one liter per minute in a conduit. Most specifically, it relates to the measurement of the rate of infusion of therapeutic agents to patients to achieve highly accurate dosing of these patients according to a prescribed administration regimen.
B. Related Art
Many methods of measuring the flow rate of liquids, and in particular the rate of infusion of a pharmaceutical to a patient are known. Best known are positive displacement systems wherein a known volume of fluid is moved over time independent of other system parameters such as pressure and liquid viscosity. Today, the most commonly used positive displacement pump for accurate infusion of a pharmaceutical to a patient is the syringe pump. A motor moves a plunger down the barrel of a syringe with tightly controlled manufacturing tolerances on inside diameter. The rate of advance of the plunger times the time of advance times the cross-sectional area of the syringe determines the volume of fluid infused. This positive displacement method is used, for example, in the MiniMed Model 508 insulin pump, the current market share leader in insulin pumps. The suggested retail price for the MiniMed 508 pump is $5,995.00. A second example of a positive displacement system is the peristaltic pump, where rollers placed against a flexible conduit roll along the conduit to move the fluid down the conduit. In peristaltic pumps, enough force is applied to the liquid in the flexible conduit to eliminate any dependence on pressure and viscosity. However, the volume of liquid dispensed remains dependent on the volume of fluid in the tubing, which depends on the square of the inside diameter of the elastomeric tubing. Since the manufacturing tolerance on the inside diameter of economic elastomeric tubing is on the order of +/−10%, the delivery accuracy is limited to +/−20%. Peristaltic pumps are also expensive, but somewhat less expensive that syringe pumps. Today, peristaltic pumps are seldom used for accurate delivery of pharmaceuticals.
Given the expense of these positive displacement pumps, and the need to find less expensive systems for accurate delivery of pharmaceuticals, many other devices and methods have been proposed to maintain the required level of accuracy while reducing the cost. It is clear that many of these proposed systems achieve the goal of reduced expense. However, the problem that these proposed schemes face is that they do not achieve the required accuracy of delivery of the pharmaceutical. For example, in a liquid dispensing system with a pressurized liquid container where the pressure on the liquid forces it along the conduit, the parameters dictating the flow include the pressure that is causing the liquid to flow, the inside diameter of the conduit along which the liquid is flowing, the length of the conduit, and the viscosity of the liquid, which is in turn dependent on the temperature of the liquid. This problem is further compounded by the fact that the dependence on the inside diameter of the conduit is a fourth power dependence. In many delivery systems of this type, the pressure on the liquid decreases as the amount of liquid in the container decreases, leading to a reduction in the flow rate. The solution to this pressure decrease is known. O'Boyle in U.S. Pat. No. 4,874,386, teaches a liquid dispensing device that accurately controls the pressure in this type of dispensing system by incorporating a constant pressure spring. But the dimensions of the flow conduit, its cross section, and the temperature for viscosity control are left uncontrolled, with the result of inaccurate dispensing of the fluid.
In order to overcome the situation of having to manufacture dispensing system components to higher tolerances than is economically feasible, many methods of measuring the liquid flow rate have been taught. If the actual flow rate is measurable, the flow rate may be adjusted to the desired flow rate. Or, if an accurate total volume rather than flow rate is required, the required time of flow may be calculated using the actual flow rate to achieve the desired volume.
In general, the different types of liquid flow measuring systems can be divided into two classes—those that require contact with the liquid to measure the flow, and those that measure the flow without requiring contact with the liquid. Flow measuring systems in the first class include a) turbines, where the angular speed of the propeller in the stream is a measure of flow rate, b) pressure drop systems, where the pressure difference across a flow resistor is used to calculate the flow rate, and c) certain forms of “thermal time of flight” systems where elements that add heat to the stream and measure heat in the stream are used to measure flow rate. Examples of these “thermal time of flight” systems are taught by Miller, Jr. in U.S. Pat. No. 4,532,811 and by Jerman in U.S. Pat. No. 5,533,412. However, in many liquid delivery systems, the conduit along which the liquid flows requires frequent replacement and, in the case of pharmaceutical infusion systems, the total flow path must also be kept sterile. In this first class of types of flow meters, the added complexity of adding components, and their necessary leads and connectors to the replaceable conduits, causes the replacement conduits to be expensive. And if these additional components are added to a reusable portion of the dispensing system, the replacement of the liquid container, or addition of fresh liquid to an existing container opens the flow path to an unsterile environment. For these reasons, attention has been paid to the invention of the second class of flow meters—those that do not require contact with the liquid in the conduit and add complexity to the conduit.
Kerlin, Jr, in U.S. Pat. No. 4,777,368, teaches a method and apparatus for non-contact measurement of the velocity of a moving mass. In a preferred embodiment, an infrared heat source raises the temperature of an element of the moving mass and an infrared detector, viewing this element of the moving mass at a later time, detects the heated element and records the time required for the moving mass to move from heater to detector. Given the physical separation of the heater and the detector, the speed of the moving mass may be calculated. Kerlin makes reference to the use of this concept for liquids as well as solids. Goldberg, in U.S. Pat. No. 4,938,079 teaches the same basic concept as Kerlin, Jr. with the modification that microwave energy is used to heat the liquid within a conduit and a microwave detector is used to sense the heated liquid downstream from the heater. Frank et al in U.S. Pat. No. 5,211,626 also teaches a thermal time of flight flow metering method, and while at least one infrared detector is used to detect the heated liquid, the liquid is heated by thermal contact with the liquid through the wall of the conduit. Taught by Goldberg and Frank is the need for accurate delivery of the liquid (although Frank admits that his teachings apply only to relatively non-accurate delivery of the liquid) the need for a closed flow path, and the need for an inexpensive replaceable conduit. However, each of these patents fails to recognize that the measured time of flight and the calculated stream velocity are insufficient to completely correct for variations in system components. Flow rate, as measured in volume per unit time, requires not only a measurement of the time, but also the volume of liquid dispensed in that time. Or, if the measurement of velocity is made, as described in all three of these teachings, then to obtain the flow rate, the cross-sectional area of the conduit must be known. These above three teachings teach the measurement of time only. The volume component is critically dependent on the inside diameter of the conduit. If time is the measured parameter, then flow rate depends on the cross-sectional area of the column of liquid and the length of the column of liquid. The cross-sectional area depends on the square of the inside diameter of the conduit. As described above, typical tolerances on the inside diameter of a conduit, especially for conduits of relatively small inside diameter, are +/−10%. Hence the variation in volumetric flow rate, even given a perfectly accurate measurement of the time of flight, is +/−20%. And if the liquid velocity is calculated from the time of flight, the flow rate depends on the cross-sectional area, which, as described above, leads to a flow rate uncertainty of +/−20%. And in situations where the conduit is to be replaced frequently, unless the inside diameter of the conduit is measured by the device or measured in the factory and communicated to the device, both expensive steps, the uncertainty due to the unknown inside diameter remains. A device and method that achieves accurate measurement of flow rate, and hence accurate liquid delivery by compensating for both variations in the inside diameter of the conduit and the velocity of the liquid flowing in the conduit is disclosed in pending U.S. application Ser. No. 09/867,003 filed May 29, 2001. This application is incorporated herein by reference.
In the teachings of U.S. Pat. Nos. 4,777,368, 4,938,079, and 5,211,626 there are additional practical considerations that make these teachings difficult to reduce to practice in cost-effective commercial products. The first of these practical aspects is the heating of the portion of the liquid to be sensed. Due to the high heat capacity and the rapid thermal diffusivity of virtually all liquids of commercial importance, and especially water, which is the base of all pharmaceutical infusion fluids, heating the liquid fast enough and to a high enough temperature to realize an operation flow meter is very difficult. Kerlin, Jr. in U.S. Pat. No. 4,777,368 implicitly recognizes this by advocating a high power CO2 laser. Neither Frank in U.S. Pat. No. 5,211,626 nor Goldberg, in U.S. Pat. No. 4,938,368 recognize this problem. And the problem is especially acute for Frank since his teachings require the heat to pass through the wall of the conduit by conduction, which is especially time-consuming and lossy. A solution to this problem, which is not alluded to in any of these three teachings, is to stop the flow of the liquid and to heat the liquid while it is stationary. The flow rate is measured by restarting flow once the liquid is heated. The two advantages of stopping the flow to heat the liquid is that the total mass of liquid that must be heated is greatly reduced and the heat pulse is relatively confined in position along the conduit. This solution is taught in pending U.S. application Ser. No. 09/867,003.
The second practical aspect which makes the prior art teaching, including the teaching in pending U.S. application Ser. No. 09/867,003, difficult to commercialize is the mode of detecting the heat pulse. Many pharmaceutical solutions, especially protein solutions such as insulin, degrade at temperatures above room temperature, and begin to denature at temperatures above 40 deg centigrade. A preferred temperature rise would be less than 5 centigrade degrees above ambient. For these systems to operate successfully the heated portion of liquid must be accurately detected and its location along the conduit accurately measured. Detection methods relying on detecting the infrared radiation from such a small change in temperature, such as proposed by Frank in U.S. Pat. No. 5,211,626, Kerlin, Jr. in U.S. Pat. No. 4,938,079, and Sage, Jr. in pending U.S. application Ser. No. 09/867,003, must operate in the far infrared where detectors are either too slow to respond to the heated liquid or must be cooled, making them large, energy consuming and expensive. Goldberg in U.S. Pat. No. 4,938,079 is sensitive to this issue, but offers no data to support a practical or operational device.
Thus there continues to be a need for improved devices and methods for accurate and economical measurement of liquid flow in liquid dispensing systems, especially in the area of infusion of pharmaceutical solutions. This invention meets these needs.
An object of the current invention is to provide an accurate, inexpensive, and practical system and method for measuring the volumetric flow of a liquid in a conduit.
It is a further object of the current invention to use this system and method for measuring the volumetric flow of a liquid in a conduit to infuse pharmaceutical solutions. This flow rate may be used for either accurate delivery of the pharmaceutical solutions or, when zero flow rate is measured, to detect occlusions in a delivery system for pharmaceutical solutions.
It is yet another object of the current invention to provide an accurate, inexpensive and practical system and method for detecting and measuring the temperature of a liquid in a conduit.